1. Field of the Invention
The present invention relates generally to a focusing control apparatus and method, and more particularly, to a fast and accurate focusing control apparatus and method.
2. Description of the Prior Art
With the coming of digital era, a lot of digital apparatuses have been widely applied in the modern life. Wherein, digital image recording equipment, such as digital camera (DC) or digital video recorder (DV), has gradually replaced traditional image recording equipment. Moreover, digital image recording system has been disposed in many portable electronic apparatus, such as mobile phones, personal digital assistants (PDA), or notebooks, so as to allow the users to capture images any time any where.
In the past, the resolution of the digital image recording system applied in most of the above-mentioned portable electronic apparatus is less than mega-pixel. Because the image quality of the portable electronic apparatus is not determined by the focusing apparatus, most digital image recording systems of the portable electronic apparatuses apply prime lens rather than zoom lens. With the increase of sensor resolution, the quality of image is more emphasized than before. Accordingly, zoom lens is gradually used in the digital image recording systems of the portable electronic apparatuses.
Please refer to FIG. 1, which illustrates a functional block of a digital image recording system of the prior art. As shown in FIG. 1, the digital image recording system 7 includes a lens 72, an image sensor 74, a processing module 76 and a motor 78. The image sensor 74, such as a CCD or a CMOS, can sense image through the lens 72, and transfer the sensed image to the processing module 76. The processing module 76 can perform noise inhibition, image sharpness evaluation and other suitable processes to the image, and output the processed image. Additionally, the processing module 76 can further generate a control signal based on the image, and can send the control signal to the motor 78, such that the motor 78 can adjust the position of the lens 72, according to the control signal, to focus for better focusing.
In the prior art, the focusing process of the digital image recording system can mainly be divided into two steps: image sharpness evaluation and focus search. The image sharpness evaluation is used to determine the clarity of the image, and the focus search is used to move the lens to suitable focus position. Additionally, it is important to find the ways to focus quickly and efficiently when the digital image recording system is applied in the above-mentioned portable electronic apparatus.
Currently, many image sharpness evaluation algorithms have been applied in digital cameras, however, most of the algorithms are too complicated to be applied in the above-mentioned portable electronic apparatus. Moreover, one of the most popular focus searching algorithms is mountain climbing search method. The mountain climbing search method needs the motor to move the lens forward and backward to perform a coarse search which is followed by a fine search.
During the coarse search process, the motor moves the lens to each focusing position in a larger scale to obtain a focus value of each focusing position. When the slope of the curve constructed by the focus values is changed, the fine search process is initiated. During the fine search process, the motor moves back the lens in a smaller scale to the focusing positions with the maximum focus value. However, the focus searching algorithms needs the motor to move the lens forward and backward, it takes a lot of time to find the focusing position and increases the consumption of the motor and related driving elements.